Purpose: The impact of conversion on perioperative and long-term oncologic outcomes is controversial. Thus, we compared these outcomes between laparoscopic (Lap), unplanned conversion (Conversion), and planned open (Open) liver resection for hepatocellular carcinoma (HCC) located in anterolateral (AL) liver segments and aimed to identify risk factors for unplanned conversion. Methods: We retrospectively studied 374 patients (Lap, 299; Open, 62; Conversion, 13) who underwent liver resection for HCC located in AL segments between 2004 and 2018. Results: Compared to the Lap group, the Conversion group showed greater values for operation time (p < 0.001), blood loss (p = 0.021), transfusion rate (p = 0.009), postoperative complication rate (p = 0.008), and hospital stay (p = 0.040), with a lower R0 resection rate (p < 0.001) and disease-free survival (p = 0.001). Compared with the Open group, the Conversion group had a longer operation time (p = 0.012) and greater blood loss (p = 0.024). Risk factors for unplanned conversion were large tumor size (odds ratio [OR], 1.35; 95% confidence interval [CI], 1.05–1.74; p = 0.020), multiple tumors (OR, 5.95; 95% CI, 1.45–24.39; p = 0.013), and other organ invasion (OR, 15.32; 95% CI, 1.80–130.59; p = 0.013). Conclusion In conclusion, patients who experienced unplanned conversion during LLR for HCC located in AL segments showed poor perioperative and long-term outcomes compared to those who underwent planned laparoscopic and open liver resection. Therefore, open liver resection should be considered in patients with risk factors for unplanned conversion.